Boil dry detection in cooking appliances

ABSTRACT

Boil dry conditions are detected in utensils heated on a glass-ceramic cooking appliance having at least one heating unit disposed under a glass-ceramic plate by monitoring the level of power supplied to the heating unit. A power level detector is electrically connected to the heating unit and generates a signal representative of the power level. A controller for controlling the heating unit so as to prevent the glass-ceramic plate from exceeding a maximum temperature is arranged to receive the power signal. The controller provides a boil dry indication in response to a decrease in the power signal. Alternatively, the controller can monitor the level of power being supplied to the heating unit by monitoring a signal generated to control the level of power, thereby eliminating the power level detector.

BACKGROUND OF THE INVENTION

This invention relates generally to detecting a boil dry condition in autensil being heated on a cooking appliance and more particularly toboil dry detection in glass-ceramic cooking appliances.

The use of glass-ceramic plates as the cooking surface in cookingappliances such as cooktops and ranges is well known. Such cookingappliances (referred to herein as glass-ceramic cooktop appliances)typically include a number of heating units mounted under theglass-ceramic plate, one or more sensors for measuring the glass-ceramictemperature, and an electronic or electro-mechanical controller. Theglass-ceramic plate presents a pleasing appearance and is easily cleanedin that its smooth, continuous surface lacks seams or recesses in whichdebris can accumulate. The glass-ceramic plate also prevents spilloversfrom falling onto the heating units below. The controller controls thepower applied to the heating units in response to user input and inputfrom the temperature sensors.

In one known type of glass-ceramic cooktop appliance, the glass-ceramicplate is heated predominantly by radiation from one or more of theheating units disposed beneath the plate. The glass-ceramic plate issufficiently heated by the heating unit to heat utensils placed on itprimarily by conduction from the heated glass-ceramic plate to theutensil. Another type of glass-ceramic cooktop appliance uses a heatingunit that radiates substantially in the infrared region in combinationwith a glass-ceramic plate that is substantially transparent to suchradiation. In these appliances, a utensil placed on the cooking surfaceis heated partially by radiation transmitted directly from the heatingunit to the utensil, in addition to conduction from the glass-ceramicplate. Such radiant glass-ceramic cooktop appliances are more thermallyefficient than other glass-ceramic cooktop appliances and have thefurther advantage of responding more quickly to changes in the powerlevel applied to the heating unit. Yet another type of glass-ceramiccooktop appliance inductively heats utensils placed on the cookingsurface. In this case, the heating unit is a coil connected to an RFgenerator; the coil emits RF energy when activated. The utensil, whichcomprises an appropriate material, absorbs the RF energy and is thusheated.

In each type of glass-ceramic cooktop appliances, provision must be madeto avoid overheating the glass-ceramic plate. For most glass-ceramicmaterials, the operating temperature should not exceed approximately600-700°C. for any prolonged period. Under normal operating conditions,the temperature of the glass-ceramic plate will generally remain belowthis limit. However, conditions can occur which can cause thistemperature limit to be exceeded. Commonly occurring examples includeoperating the appliance with a small load or no load (i.e., no utensil)on the cooking surface, using badly warped utensils that make unevencontact with the cooking surface, and operating the appliance with ashiny and/or empty utensil.

To protect the glass-ceramic plate from extreme temperatures, a controlsystem is utilized in which temperature sensors provide a signalindicative of the glass-ceramic temperature to the appliance'scontroller. If the glass-ceramic plate approaches its maximumtemperature, a special control mode, known as the thermal limiter mode,is activated. In the thermal limiter mode, the controller reduces powerto the heating units to maintain the temperature of the glass-ceramiccooking surface at a relatively constant, safe temperature.

Another concern with cooking appliance generally is a boil drycondition. A boil dry condition occurs when all the liquid contents of aheated utensil evaporate during the boil phase. This commonly happenswhen a utensil is inadvertently left on a hot cooking surface orotherwise overheated. A boil dry condition can cause burned food,utensil damage and potential fire hazards. Accordingly, automaticdetection of a boil dry condition is a desirable feature in cookingappliances.

In glass-ceramic cooktop appliances, it is known to use the glassceramic temperature to determine when a utensil has boiled dry.Specifically, when a utensil containing water or another liquid isplaced on a glass-ceramic cooking surface and the burner is turned on,the glass-ceramic temperature initially increases rapidly. Theglass-ceramic temperature will continue to rise until the utensilcontents come to a boil. During the boil phase, the utensil contentswill boil off at a steady temperature and remove excess heat viaevaporation. With this steady heat removal, the glass-ceramictemperature also reaches a steady state value some time after thecontents come to a boil. However, when the liquid completely boils off,there is a sudden drop in heat removal from the pan, and consequently,the glass-ceramic temperature increases rapidly. This temperature riseis thus indicative of the boil dry condition.

This method of boil dry detection generally works well while the cookingappliance is in its standard operating mode. But under the thermallimiter mode, the glass-ceramic plate is being maintained at arelatively constant temperature by the controller. Therefore, theglass-ceramic temperature will not rise when upon a boil dry condition.Accordingly, it would be desirable to be able to automatically detectboil dry conditions while in the thermal limiter mode.

BRIEF SUMMARY OF THE INVENTION

The above-mentioned need is met by the present invention, which providesa boil dry detection system for a glass-ceramic cooking appliance havingat least one heating unit disposed under a glass-ceramic plate and apower source for providing power to the heating unit. The boil drydetection system includes means for providing a signal representative ofthe level of power being supplied to the heating unit. A controller forcontrolling the power source so as to prevent the glass-ceramic platefrom exceeding a maximum temperature is arranged to receive the powerlevel signal. The controller provides a boil dry indication in responseto a decrease in the power level signal.

The present invention and its advantages over the prior art will becomeapparent upon reading the following detailed description and theappended claims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a perspective view of a glass-ceramic cooktop applianceincorporating a preferred embodiment of the present invention.

FIG. 2 is partly schematic view of a glass-ceramic cooktop applianceshowing one of its burner assemblies in cross-section.

FIG. 3 is a graph plotting glass-ceramic temperature as a function oftime.

FIG. 4 is a graph plotting power level as a function of time.

FIG. 5 is a partly schematic view of a glass-ceramic cooktop applianceshowing an alternative embodiment of a burner assembly in cross-section.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 shows aglass-ceramic cooktop appliance 10 having a glass-ceramic plate 12 thatprovides a cooking surface. The appliance 10 can be any type of cooktopappliance including a range having an oven and a cooktop providedthereon or a built-in cooktop unit without an oven. Circular patterns 14formed on the cooking surface of the plate 12 identify the positions ofeach of a number (typically, but not necessarily, four) of burnerassemblies (not shown in FIG. 1) located directly underneath the plate12. A control panel 16 is also provided. As is known in the field, thecontrol panel 16 includes knobs, touch pads or the like that allow anoperator of the appliance 10 to individually control the power appliedto the burner assemblies.

Turning to FIG. 2, an exemplary one of the burner assemblies, designatedgenerally by reference numeral 18, is shown located beneath theglass-ceramic plate 12 so as to heat the plate 12 and/or a utensil 15placed thereon. The burner assembly 18 includes a controllable heatingunit 20 in the form of an open coil electrical resistance element, whichis designed when fully energized to radiate primarily in the infraredregion of the electromagnetic energy spectrum. It should be noted thatother types of heating units could be used in place of the resistanceelement. The heating unit 20 is arranged in an effective heating patternsuch as a concentric coil and is secured to the base of an insulatingliner 22 which is supported in a sheet metal support pan 24. Theinsulating liner 22 includes an annular, upwardly extending portion 26that serves as an insulating spacer between the heating unit 20 and theglass-ceramic plate 12. The support pan 24 is typically spring loadedupwardly, forcing the annular portion 26 into abutting engagement withthe underside of the glass-ceramic plate 12, by conventional supportmeans (not shown).

The heating unit 20 is coupled to a power source 28 (typically astandard 240 volt, 60 Hz AC power source) via suitable power lines 30. Apower source control means such as a triac 32 is provided to regulatethe level of power delivered to the heating unit 20. The triac 32 is aconventional semiconductor device capable of conducting current ineither direction across its main terminals when triggered by either apositive or negative voltage or signal 34 applied to its gate terminal36. An electronic controller 38 supplies the gate signal 34. Thecontroller 38 controls the power applied to the heating unit 20 bycontrolling the rate at which gate signals 34 are applied to the triacgate terminal 36. The gate signal pulse rate is dictated by the powersetting selections for the burner assembly 18 entered by user actuationof the control panel 16. Although not shown in FIG. 2, other heatingunits included in the appliance 10 are connected to the power source 28in the same manner as, and in parallel with, the illustrated heatingunit 20.

A power level detector 40 is electrically connected to the heating unit20, preferably between the power source 28 and the heating unit 20, andgenerates a signal 42 that is directly or indirectly representative ofthe amount of power applied to the heating unit 20. In this sense, thepower level detector 40 does not necessarily detect power directly, butis referred to herein as a “power level detector” because it providesthe system with the means to detect the power level. The power levelsignal is fed to the controller 38. In one preferred embodiment, thepower level detector 40 is any suitable circuit or device capable ofmeasuring RMS voltage and generating a signal corresponding to themeasured voltage. This controller 38 uses the measured voltage and theresistance of the heating unit 20 to determine the power level appliedto the heating unit 20. Thus, the power signal 42 would be indirectlyrepresentative of the power level.

For purposes of the power calculation, the heating unit resistance isassumed to be constant. Although the heating unit resistance actuallyvaries significantly over the entire operating range of the appliance10, this resistance is fairly constant while the appliance is in itsthermal limiter mode, which is described below. As also described below,the controller 38 uses the power signal primarily during thermal limitermode operation. Thus, the assumption of a constant heating unitresistance is valid for the power calculation. This value of the heatingunit resistance used in the calculation is a predetermined value that isbased on a mean resistance value typically provided by the manufacturer.

Alternatively, the power level detector 40 could comprise conventionalcircuitry designed to measure the instantaneous current andinstantaneous voltage and use these values to determine the power level.In this case, the power signal 42 is directly representative of thepower level. However, such circuitry is generally more expensive.

A temperature sensor 44 is provided to detect the temperature of theglass-ceramic plate 12. In one preferred embodiment, the temperaturesensor 44 is a resistive element, such as a resistance temperaturedetector, having a resistance that is very sensitive to temperature. Theresistive element 44 is placed within the heater cavity 46 defined bythe insulating liner 22 and the glass-ceramic plate 12. Alternatively,the temperature sensor 44 could be a resistance temperature detector orsimilar device attached directly to the underside of the glass-ceramicplate 12. In any event, the temperature sensor 44 generates a signal 48indicative of temperature that is fed to the controller 38.

During normal operation, a user selects the desired cooking setting viamanipulation of the control panel 16, and the controller 38 suppliesgate signals 34 to the triac gate terminal 36 at an appropriate rate soas to provide the necessary level of power from the power source 28 tothe heating unit 20. However, overheating of the glass-ceramic plate 12should be avoided to insure long life. Thus, the controller 38 monitorsthe temperature signal 48 provided by the temperature sensor 44 toinsure that the glass-ceramic temperature does not exceed a maximum safelevel. Specifically, as the utensil 15 is being heated, the temperatureof the glass-ceramic plate 12 will generally increase. If theglass-ceramic temperature reaches a preset value, which is typically inthe range of 600-700° C., then the controller 38 will activate itsthermal limiter mode to protect the glass-ceramic plate 12 fromoverheating. Under the thermal limiter mode, the controller 38 controlsthe pulse rate of the gate signals 34 such that the power supplied tothe heating unit 20 is reduced to maintain the glass-ceramic temperaturebelow the maximum safe level. Accordingly, the glass-ceramic temperatureis maintained at a relatively constant level during the thermal limitermode.

The controller 38 also provides a boil dry detection function. Asmentioned previously, a boil dry condition occurs when all the liquidcontents of a heated utensil are boiled off. During normal operation,the controller 38 detects a boil dry condition based on temperaturesignal 48. This is illustrated by referring to FIG. 3, which shows aplot of the temperature signal 48 as a function of time. The utensil 15is placed on the glass-ceramic plate 12 and the appliance 10 is turnedon, at time t₀, causing the glass-ceramic temperature to increase fromroom temperature. The glass-ceramic temperature continues to rise untilthe utensil contents come to a boil. During the boil phase, the utensilcontents will boil off at a steady temperature and remove excess heatvia evaporation. With this steady heat removal, the glass-ceramictemperature also reaches a steady state value at time t₁, which is ashort time after the contents have come to a boil. If the heating iscontinued, the liquid contents will eventually completely boil off, asshown at time t₂. At this point, there is a sudden drop in heat removalfrom the utensil 15, and consequently, the glass-ceramic temperatureincreases rapidly. This rise in the temperature signal 48 is thusindicative of the boil dry condition. In response to detecting a boildry condition, the controller 38 shuts off power to the heating unit 20and optionally sends a triggering signal to an alarm 50 (FIG. 2).

In the thermal limiter mode, the temperature signal 48 is maintained ata steady state. This means that the temperature signal 48 will not riseif a boil dry condition occurs while the appliance 10 is operating underthe thermal limiter mode. The controller 38 provides boil dry detectionduring the thermal limiter mode by monitoring the level of power appliedto the heating unit 20 via the power signal 42 provided by the powerlevel detector 40. Referring to FIG. 4, which shows a plot of the powersignal 42 as a function of time, time t₀ again represents the point atwhich the utensil 15 is placed on the glass-ceramic plate 12 and theappliance 10 is turned on. The power level is determined by the desiredcooking setting selected by the user via manipulation of the controlpanel 16 and generally remains constant as long as the cooking settingis unchanged by the user. The glass-ceramic temperature will increaseand at some point, represented by time t_(L) in FIG. 4, can reach thepreset value causing the controller 38 to activate the thermal limitermode. At this point, the controller 38 will reduce the power levelsupplied to the heating unit 20 so as to maintain the glass-ceramicplate 12 at a safe temperature.

At some point in the heating process (which could be either before ofafter the time t_(L) when the thermal limiter mode is activated), theutensil contents come to a boil. During the boil phase, the utensilcontents will boil off at a steady temperature and remove excess heatvia evaporation at a steady rate. With this steady heat removal, thepower level supplied to the heating unit 20 in order to maintain theglass-ceramic temperature at its safe level will generally remainsteady, although there may be slight fluctuations in the power level dueto changes in room temperature and the like. Continued heating willresult in the liquid contents eventually being completely boiled off, asshown at time t₂. At this point, there is a sudden drop in heat removalfrom the utensil 15 meaning less power is required to maintain theglass-ceramic temperature. Therefore, the power signal 42 will show anabrupt drop that will be indicative of the boil dry condition. Asbefore, the controller 38 will shut off power to the heating unit 20 andoptionally send a signal to an alarm 50. Generally, the abrupt drop inthe power signal 42 need be only on the order of about 2-3% over aperiod of a few seconds to trigger a boil dry indication.

FIG. 5 shows an alternative embodiment in which boil dry detection inthe thermal limiter mode is accomplished by monitoring the level ofpower applied to the heating unit. However, in this embodiment, aseparate power level detector is not used. Specifically, FIG. 5 shows aburner assembly 118 located beneath a glass-ceramic plate 112 so as toheat the plate 112 and/or a utensil 115 placed thereon. The burnerassembly 118 includes a controllable heating unit 120, which is the sameor similar to that of the first described embodiment. The heating unit120 is coupled to a power source 128 (typically a standard 240 volt, 60Hz AC power source) via suitable power lines 130.

A power source control means such as a triac 132 is provided to regulatethe level of power delivered to the heating unit 120. The triac 132 is aconventional semiconductor device capable of conducting current ineither direction across its main terminals when triggered by either apositive or negative voltage or signal 134 applied to its gate terminal136. An electronic controller 138 supplies the gate signal 134. Thecontroller 138 controls the power applied to the heating unit 120 bycontrolling the rate at which gate signals 134 are applied to the triacgate terminal 136. The gate signal pulse rate is dictated by the powersetting selections for the burner assembly 118 entered by user actuationof the control panel 116. A temperature sensor 144, which is the same asor similar to the temperature sensor of the first described embodiment,is provided to detect the temperature of the glass-ceramic plate 112. Inany event, the temperature sensor 144 generates a signal 148 indicativeof temperature that is fed to the controller 138.

Boil dry detection during normal operation is accomplished in the samemanner as described above. That is, the controller 138 monitors thetemperature signal 148 where a rapid rise in the temperature signal 144is indicative of a boil dry condition. Boil dry detection during thethermal limiter mode is accomplished monitoring the level of powerapplied to the heating unit 120, wherein an abrupt drop in the powerlevel is indicative of a boil dry condition. Instead of monitoring thepower level with a power level detector, the controller 138 monitors thegate signal pulse rate, which is controlled by, and thus “known” by, thecontroller 138. Because the power level is a function of the gate signalpulse rate, the gate signal 134, like the power signal 42 of the firstdescribed embodiment, is representative of the level of power beingsupplied to the heating unit 120. As before, the controller 138 shutsoff power to the heating unit 120 and optionally sends a triggeringsignal to an alarm 150 in response to detecting a boil dry condition.

The foregoing has described a method and system for automaticallydetecting boil dry conditions, including monitoring the power level todetect boil dry conditions while operating in the thermal limiter mode.While specific embodiments of the present invention have been described,it will be apparent to those skilled in the art that variousmodifications thereto can be made without departing from the spirit andscope of the invention as defined in the appended claims.

What is claimed is:
 1. A boil dry detection system for a cooking appliance having at least one heating unit, said system comprising: means for providing a signal representative of a level of power being supplied to said heating unit; and means for monitoring said signal for boil dry detection, said means for monitoring said signal providing a boil dry indication in response to a decrease in said signal.
 2. The boil dry detection system of claim 1 wherein said means for providing a signal comprises a power level detector electrically connected to said heating unit.
 3. The boil dry detection system of claim 2 wherein said power level detector measures voltage and said controller determines said level of power being supplied to said heating unit from said measured voltage and a predetermined resistance value of said heating unit.
 4. The boil dry detection system of claim 2 wherein said power level detector measures an instantaneous current and voltage and determines said level of power being supplied to said heating unit from said measured instantaneous current and voltage.
 5. The boil dry detection system of claim 1 wherein said means for providing a signal controls said level of power being supplied to said heating unit.
 6. The boil dry detection system of claim 1 wherein said means for monitoring said signal controls said level of power being supplied to said heating unit.
 7. The boil dry detection system of claim 6 wherein said means for monitoring said signal shuts off power to said heating unit in response to a decrease in said signal.
 8. The boil dry detection system of claim 1 wherein said means for monitoring said signal triggers an alarm in response to a decrease in said signal.
 9. A boil dry detection system for a glass-ceramic cooking appliance having at least one heating unit disposed under a glass-ceramic plate and a power source for providing power to said heating unit,. said system comprising: means for providing a signal representative of a level of power being supplied to said heating unit; and a controller controlling said power source so as to prevent said glass-ceramic plate from exceeding a maximum temperature, said controller monitoring said signal and providing a boil dry indication in response to a decrease in said signal.
 10. The boil dry detection system of claim 9 wherein said means for providing a signal comprises a power level detector electrically connected to said power source.
 11. The boil dry detection system of claim 10 wherein said power level detector measures voltage and said controller determines said level of power being supplied to said heating unit from said measured voltage and a predetermined resistance value of said heating unit.
 12. The boil dry detection system of claim 10 wherein said power level detector measures an instantaneous current and voltage and determines said level of power being supplied to said heating unit from said measured instantaneous current and voltage.
 13. The boil dry detection system of claim 9 wherein said controller shuts off power to said heating unit in response to a decrease in said signal.
 14. The boil dry detection system of claim 9 wherein said controller triggers an alarm in response to a decrease in said signal.
 15. A method of detecting a boil dry condition in a utensil being heated on a cooking appliance having at least one heating unit, said method comprising the steps of: monitoring a level of power being supplied to said heating unit; and providing a boil dry indication in response to a decrease in said level of power being supplied to said heating unit.
 16. The method of claim 15 wherein said step of monitoring a level of power being supplied to said heating unit comprises measuring a voltage applied to said heating unit and determining said level of power being supplied to said heating unit from said measured voltage and a predetermined resistance value of said heating unit.
 17. The method of claim 15 wherein said step of monitoring a level of power being supplied to said heating unit comprises measuring an instantaneous current and voltage and determining said level of power being supplied to said heating unit from said measured instantaneous current and voltage.
 18. The method of claim 15 wherein said step of monitoring a level of power being supplied to said heating unit comprises monitoring a signal generated to control said level of power being supplied to said heating unit.
 19. The method of claim 15 further comprising the step of shutting off power to said heating unit in response to a boil dry indication.
 20. The method of claim 15 further comprising the step of triggering an alarm in response to a boil dry indication. 